The use of abrasive belts in machines for surfacing lumber is well-known, being taught for example in Schuster U.S. Pat. No. 3,872,627. When surfacing lumber of less width than the abrasive belts themselves, efficiency is served if several workpieces can pass through the machine in side-by-side relationship, both faces of each piece being surfaced at the same time and with removal of substantially the same thickness from each surface of the work, thereby finishing all workpieces to the same ultimate thickness. This results in some difficulty, however because as supplied from the sawmill, unfinished lumber workpieces not only vary considerably in thickness, but are frequently bowed or curved transversely, either of which can interfere with proper feed of the pieces through the machine. The device of the present invention permits the unfinished lumber workpieces to be treated on opposed surfaces with both such surfaces receiving cuts of substantially the same thickness.